1. Field of Invention
This invention relates to heating agents that heat in alternating magnetic fields, and more specifically to fibers that generate unexpectedly high heating rates and do so despite being smaller in diameter than previously thought necessary to generate significant heating.
2. Description of Prior Art
Radio frequency alternating magnetic fields have been used for some time to generate heat in heating agents. The process is related to and uses equipment similar to the induction heating generators used in the heat treating of metals. The heating agents are electrically conductive and/or magnetic. An electrically non-conductive, non-magnetic material or matrix is transparent to a magnetic field in the radio frequency range and therefore cannot be heated by the field. The heating agents are normally added to or placed upon or in materials or matrices that would otherwise not heat or not heat efficiently in an alternating field of a given frequency and intensity (the resulting combinations hereafter termed "heating matrices"). Applications include: bonding or welding of thermoplastics; curing of thermosets; melting or curing of adhesives e.g. thermoplastics, thermosets, thermoplastic/thermosets, elastomerics, etc.; activating foaming agents; initiating polymerization; curing ceramics; generation of heat in containers, inserts or tooling which, in turn, transfer heat to materials in thermal contact therewith; starting or accelerating catalytic reactions; heat sealing, compression and transfer molding and numerous other applications.
The frequencies utilized in practical applications range from: 50 KHz+ for the heating of metal screens; to 2-5 MHz for the heating of ferromagnetic particles e.g. fine iron oxides; to 5-30 MHz for the heating of iron and other ferromagnetic particles. These processes are distinguished from dielectric and microwave heating. Dielectric heating operates in the 27 MHz to high MHz frequency range and generates heat by exciting the electric dipoles in the dielectric material as they try to align with the rapidly alternating electric field. Microwave heating operates at frequencies from the high MHz to the GHz range where water dipoles resonate. Neither the dielectric nor the microwave processes require heating agents in order to generate heat as long as the materials to be heated are sufficiently polar. Heating agents have been added to materials for use in both dielectric and microwave processes, where the heat needed to be concentrated or intensified. The very high frequencies involved allowed the use of very small particles that would not heat as efficiently, if at all, at lower frequencies.
Heat is predominantly generated in heating agents by either hysteresis or eddy current losses. Hysteresis losses occur in any magnetic material. Magnetic dipoles within each magnetic domain of the particle attempt to align themselves with the rapidly alternating magnetic field. The energy required to rotate them is dissipated as heat, the rate at which the heat is generated increases with the rate of reversal of the magnetic field--i.e. the frequency of the alternating current. The hysteresis loop differs for each magnetic material and depends upon the strength of the magnetic field and the properties of the material. The area within the hysteresis loop reflects the magnitude of the hysteresis losses, which are manifested as heat. As long as the particle size is larger than one magnetic domain, hysteresis losses do not depend on particle size. Hysteresis occurs in non-conductive ferrimagnetic materials such as oxides and ferrites as well as ferromagnetic materials.
Eddy currents, as the name implies, are circulating currents that appear to flow in swirls or eddies in electrically conductive materials, which need not be magnetic and thus include copper and aluminum, for example. Eddy currents, like other electrical currents, require a complete electrical path. For a given eddy current there is an associated voltage drop V, which, for a pure resistance R, is given by Ohm's Law, V=IR, where I denotes current. When a voltage drop occurs, electrical energy is converted into thermal energy or heat. Eddy current heating is based on P=I.sup.2 R, thus the (P) power (heat) in watts is proportional to the square of the (I) current in amps and to the (R) resistance in ohms. Ferromagnetic materials can have both eddy current and hysteresis losses however, if the material is large enough to allow the flow of eddy currents, the heat generated by the eddy currents is generally greater than the heat generated by hysteresis.
Whether a heating agent is large enough to be heated by eddy currents is determined by its size relative to its reference depth. When a solid round bar is placed in a solenoid coil, the alternating current in the coil induces current in the bar. The bar is most easily visualized as consisting of numerous concentric sleeves. The current induced in the outermost sleeve is greater than the current induced in the second sleeve. The effective depth of the current carrying layers is the reference depth or skin depth. The reference depth is dependent on the frequency of the alternating current through the coil, and the electrical resistivity and relative magnetic permeability of the workpiece or heating agent. The definition of d is: ##EQU1## Where d is the reference depth, in inches or centimeters; p is the resistivity of the workpiece, in ohm-inches or ohm-centimeters; .mu. is the relative magnetic permeability of the workpiece or heating agent (dimensionless); and f is the frequency of the field in the work coil, in hertz. The reference depth is the distance from the surface of the material to the depth where the induced field strength and current are reduced to 1/e, or 37% of their surface value. The power density at this point is 1/e.sup.2, or 14% of its value at the surface (e=base of the natural logarithm=2.718)
As noted above, the heating efficiency of a heating agent is much higher if it is large enough for eddy currents to flow. That, in turn is determined by the electrical diameter of the workpiece or heating agent which equals aid or the ratio of the diameter of the heating agent to the reference depth. A heating agent with a ratio of &gt;4 is efficient at heating, one with a ratio of &gt;2&lt;4 is far less efficient and one with a ratio of &lt;2 is considered "no good" or unusable.
This is the basis for the definition of a "critical frequency" below which induction heating efficiency drops rapidly. Thus for a round bar the critical frequency is that at which the ratio of workpiece diameter to reference depth is approximately 4+/1. Below the critical frequency, efficiency drops rapidly because less current is induced due to current cancellation. Current cancellation becomes significant when the reference depth is such that the eddy currents induced from either side of a workpiece "impinge" upon each other and, being of opposite sign, cancel each other.
The equation for calculating the critical frequency (f.sub.c) for a round bar is: EQU f.sub.c =1.6.times.10.sup.8 p/.mu.a.sup.2 (a in in.) EQU f.sub.c =4.times.10.sup.8 p/.mu.a.sup.2 (a in cm.)
Where a=diameter of the bar; .mu.=permeability of the material making up the bar and p=resistivity in ohm-inches or ohm-centimeters of the material making up the bar.
Rather than do the calculations each time, the recommend practice in induction heating is to refer to a table or graph to determine the minimum diameters efficiently heated at a given frequency. The following example is from C. A. Tudbury, Basics of Induction Heating, John F. Rider Inc., New York, 1960.
__________________________________________________________________________ Approximate Smallest Diameter (converted to microns - .mu.) Which Can Be Heated Efficiently By The Equipment Indicated To The Temperature Shown Final Motor Spark-Gap Vacuum Tube Temp. Generator Oscillator Oscillator Material .degree. F. 3 KHz 10 KHz 50 KHz 200 KHz 450 KHz 2 MHz __________________________________________________________________________ Steel Stress relieve 1000 5,080 2,794 1,270 635 432 203 Harden 1600 38,100 21,590 9,652 4,826 3,175 1,524 __________________________________________________________________________
Thus, when making the determination of whether a given heating agent should heat efficiently at a specified frequency one can:
1) calculate the reference depth and divide the particle diameter by the reference depth to determine the electrical diameter of the heating agent; or PA1 2) calculate the critical frequency based on the particle diameter; or PA1 3) refer to a table such as the one above. PA1 Solid state generators have operating efficiencies approaching 90% in comparison with vacuum tube generators with efficiencies of 50-60%. PA1 The vacuum tube generators inefficiency manifests itself in heat which requires additional cooling capacity. PA1 Additional power is required to compensate for the inefficiency and the added cooling requirements. PA1 Vacuum tube generators are less reliable and require more expensive maintenance. PA1 Solid state generators are a fraction of the size and weight of an equivalent vacuum tube generator. In comparison with a vacuum tube generator system a compact solid state generator system of similar power could be less than 25% of its size and weight while an ultra-compact solid state generator system could be less than 10% of its size and weight. PA1 Such solid state generators could be transportable and hand-held and open up entirely new applications, particularly where in-the-field and hand-held operations are essential. PA1 1) dramatically decreasing cycle times and increasing production rates; PA1 2) coupling more effectively with an alternating magnetic field coil, thereby allowing rapid heating at greater coupling distances or shorter cycle times at equal coupling distances; PA1 3) enabling the use of smaller, less powerful, less expensive generators while producing similar cycle times; PA1 4) requiring significantly lower loading levels in the matrix yet still yielding equivalent or faster cycle times or heating rates; PA1 5) allowing the modification or enhancement of the matrix due to its requiring lower loading levels; PA1 6) allowing the matrix to be effectively transparent or easily colored due to the low loading levels; PA1 7) quickly conducting heat throughout its length, thereby evenly heating the matrix and avoiding localized hot spots and attendant matrix degradation; PA1 8) resisting migration through the matrix and therefore not damaging the matrix or increasing the potential of arcing to the coil; PA1 9) dispersing readily in the matrix and staying in suspension, thereby providing consistent loading levels and the attendant consistent heating rates; PA1 10) being readily removed from the recycling stream with standard equipment and thereby minimizing the difficulty and expense of recycling the substrate, thus decreasing manufacturing waste and cost and increasing the economics and likelihood of recycling the end product; PA1 11) absorbing high power inputs and rapidly transferring the heat without degrading the matrix, thus effectively utilizing its higher efficiency to reduce cycle times; PA1 12) effectively heating high temperature matrices due to its high efficiency and its ability to absorb high power inputs, while transferring the heat to the matrices without degrading them; PA1 13) heating efficiently and rapidly enough to reach the required temperature before the adjacent conductive substrate overheats despite being further from the coil than the substrate; PA1 14) utilizing significantly longer coils, despite their attendant lower frequencies and lower power, thereby efficiently heating larger matrices and substrates that previously could not be heated by other heating agents; PA1 15) heating efficiently at significantly lower frequencies than previously possible, thus allowing the generator to be tuned to full output power even though that results in a lower frequency; PA1 16) allowing the use of highly efficient and compact solid state generators even though they operate in the KHz frequency range instead of the MHz frequency range as required by previous heating agents; PA1 17) being corrosion resistant/non-reactive yet heating nearly as efficiently as its non-corrosion resistant counterparts and far more efficiently than the non-corrosion resistant prior art heating agents; and PA1 18) generating heat rapidly enough to bring the heating matrix and the adjoining substrate to the required temperature before the heat migrates through the substrate thereby causing distortion or degradation.
As the following factors increase the skin depth decreases, thus decreasing the minimum diameter required for efficient heating.
1) Frequency PA0 2) Permeability PA0 1) Resistivity--Thus the material with a higher resistivity has a greater skin depth. PA0 2) Temperature--As the temperature of the material rises the resistivity increases. As the temperature rises the permeability decreases. PA0 3) Power density--As the materials become magnetically saturated the permeability decreases (below the Curie temperature). PA0 1) Coils--small, closely coupled and/or multi-turn coils PA0 2) Substrates--small enough and/or of a configuration that they can be inserted in or surrounded by the coil PA0 3) Concerns--that the heating rate could be too rapid, thereby degrading the matrix PA0 1) Coils--large, relatively poorly coupled, frequently single turn PA0 2) Substrates--too large and/or of a configuration that they do not lend themselves to being inserted in or surrounded by a Category S type of coil PA0 3) Concerns--that the frequency and/or field intensity of the coil may be too low to generate sufficient heat in the heating agent and generate it rapidly enough to make the application possible or commercially viable PA0 1) Larger ferromagnetic particles generate more heat than smaller ferromagnetic particles, particularly at lower frequencies. PA0 2) As the frequency decreases the heating rates of all heating agents decrease significantly, reaching a frequency below which minimal, if any heating is observed. PA0 3) Higher loading levels of heating agents result in higher heating rates but can negatively effect the strength of the bonding agent and the bond. PA0 4) High coil currents are required to generate significant heating rates, particularly at lower frequencies. This is particularly pronounced with non-conductive ferrimagnetic heating agents. PA0 5) As illustrated by the Leatherman tests, a minimum coil current is required to generate any significant heat in any of the heating agents. At 4.5 MHz the minimum is approximately 250 Amps. At 2.8 MHz the minimum is approximately 300 Amps. PA0 6) High frequency and high current coils are increasingly difficult to engineer as the coil length increases, as required for the heating of larger bondlines or substrates.
As the following factors increase the skin depth increases, thus increasing the minimum diameter required for efficient heating.
The principles of induction heating were well known, with many of the primary texts being written in the 1930's-1950's. The shapes and compositions of the materials that would be used as heating agents were also well known. Heating agents can be conductive, non-conductive, metallic, non-metallic, ferromagnetic or ferrimagnetic. They can come in a range of shapes, for example: powders, particles, oxides, ferrites, flakes, fibers, meshes, screens, braids, foils, sheets (optionally perforated), wire loops or conductive rings. As the technology developed and the practical frequencies rose, induction heating was applied to smaller and smaller parts, particularly to heat treat or join the parts. For example, among the smallest parts that were inductively heated were wires and needles and it is on data generated in this and related areas that calculations regarding particle sizes that were suitable for induction heating were originated. It was known that for a given frequency, a conductive object would not efficiently generate heat if it was less than a specified diameter or size because eddy currents could not flow and therefore it would only generate heat by hysteresis (if it was ferromagnetic).
In the development of heating agents virtually none of the basic shapes or materials were unknown. Patentable heating agents were recognized as distinct species from a much larger genus that, under specified conditions (frequency, coupling, or coil design for example), resulted in advantageous or unexpected results. This distinguished them from the prior art though frequently the distinctions were very fine.
For example, consider a group of patents from the genus of conductive ferromagnetic particles. U.S. Pat. No. 2,087,480 to Pitman (1935) specified metal particles or filings which were added to an adhesive and which could be heated in an induction field. The adhesive could be heated (dielectrically) without the conductive particles but the addition of the metal particles allowed the heat to be generated at lower frequencies. In U.S. Pat. No. 2,393,541 to Kohler (1943) it was recognized that iron filings could be rapidly heated in an induction field by both hysteresis and eddy currents but that the iron filings became too hot and degraded the non-magnetic material they were to heat. Therefore Kohler selected ferromagnetic materials that were finely enough divided and insulated from one another by the adhesive or other dielectric material so that they only generated heat by hysteresis and not by eddy currents. When a ferromagnetic material reaches its Curie temperature its permeability drops to 1 and it ceases to generate heat by hysteresis, though it may continue to generate heat by eddy currents. By selecting the ferromagnetic material with the desired Curie temperature and specifying small particle sizes that did not heat by eddy currents, Kohler was able to specify a heating agent that had a maximum temperature below the degradation point of the matrix. In U.S. Pat. No. 3,620,875 to Guglielmo (1971) ferromagnetic particles were specified that were large enough to be heated by both hysteresis and eddy currents to above the Curie temperature.
Examples of other U.S. patents utilizing ferromagnetic particles as heating agents include: U.S. Pat. No. 4,762,864 to Goel (1988); U.S. Pat. No. 3,620,875 to Guglielmo (1971); and U.S. Pat. No. 3,477,961 to Castangna (1969). U.S. Patents with ferromagnetic heating agents limited to their Curie temperature include: U.S. Pat. No. 3,477,961 to Clark (1969); U.S. Pat. No. 5,378,879 to Monovoukas (1995); and numerous patents to McGaffigan including U.S. Pat. No. 5,126,521 (1992). Examples of metal or conductive screens, meshes or braids are found in U.S. Patents: U.S. Pat. Nos. 4,313,777 (1982) and 4,521,659 (1985) to Buckley; U.S. Pat. Nos. 5,500,511 (1996) and 5,508,496 (1996) to Hansen; and U.S. Pat. Nos. 5,313,034 (1994) and 5,481,091 (1996) to Grimm. However screens were known to heat in induction field as noted, for example, in U.S. Pat. No. 2,393,541 to Kohler (1943) and U.S. Pat. No. 3,462,336 to Leatherman (1969). Examples of carbon fiber as a heating agent include U.S. Pat. Nos.: 5,248,864 (1993) and 5,340,428 (1994) to Kodokian and U.S. Pat. No. 4,871,412 (1989) to Felix. Examples of U.S. Patents for heating agents based on hysteresis include: numerous patents assigned to Heller and invented by Heller, Leatherman or James e.g. U.S. Pat. No. 3,461,014 to James (1969) and U.S. Pat. No. 5,129,977 to Leatherman (1992); U.S. Pat. No. 5,123,989 to Horiishi (1992); and U.S. Pat. No. 3,391,846 to White (1968).
There are certain basic principles that apply to the heating of heating agents in a alternating magnetic field. They are useful in evaluating and comparing the results generated by different methodologies for varying heating agents:
1) Increasing the Current in an Induction Coil Increases the Heating Rate.
For eddy current heating the power (heat in watts) generated in a given heating agent is proportional to the square of the current induced in the heating agent (P=I.sup.2 R). Thus if the current in the coil is doubled, the current induced in the heating agent is doubled and the resultant power is 4 times as great. It was assumed that this rapid rise in heating with a rise in power only applied to heating agents heated by eddy currents, because those heated by hysteresis had no current flow. However tests run by Leatherman U.S. Pat. No. 4,969,968 (1990) suggest an even steeper rise in the heating rates of nonconductive submicron iron oxides with the increase in power. However that steep rise was only evident at very high coil currents. Leatherman was uncertain of the reason for this but postulated that it could be that with the increased magnetic field the hysteresis loop became more square, therefore its area increased and accordingly more heat was generated.
2) The Closer the Coil is to the Heating Agent the More Rapid the Heating.
The electromagnetic field varies inversely with the square of the distance between the coil and the heating agent. Thus if the distance between the coil and the heating agent is doubled the field strength is 1/4 as strong, which, in turn, induces 1/4 as much current in the heating agent. As noted above, for eddy currents power (heat) is proportional to the square of the current. If the current is reduced to 1/4, the power is reduced to 1/16.
3) Higher Frequencies Tend to Lead to More Rapid Heating of Heating Agents.
In hysteresis heating this is attributable to the greater number of times the dipoles must realign themselves per second and the heat generated with each realignment. In eddy current heating, according to Faraday's law, the voltage or electromagnetic force induced in the heating agent is proportional to the rate at which the field is changing in lines per second. Thus increasing the frequency proportionally increases the voltage induced (which in turn proportionally increases the resultant current). The effects of increased current on power (heat) generated are noted above.
4) The Intensity of the Magnetic Field is Multiplied by the Number of Turns in the Coil.
The simplest of coils is a single turn coil (FIG. 1.b) in which one lead comes out from the generator, makes a loop or turn and returns to the generator via the other lead. The solenoid coil (FIG. 1.a) is similar to the single turn coil except that it has two or more turns. The product of the current in the coil multiplied by the number of turns (ampere turns) determines the strength of the magnetic field inside the coil. Thus, for a given current a 6 turn coil has 6 times the magnetic field of a single turn coil, induces 6 times as much voltage in the same heating agent, creating eddy currents that are 6 times as strong and, because the heating agent is the same the resistance is the same, therefore the power (heat) generated is 36 times as high (P=I.sup.2 R).
5) The Smaller the Coil or the Closer the Two Legs of the Coil are the More Intense the Magnetic Field is Inside the Coil or Between the Legs.
This is due to the proximity effect. If the coil is large or the distance between the legs is large the electrical current is evenly distributed around the copper tubes or other materials that form the coil, as is the magnetic field. As the distance between the coil legs or the coil size decreases the inductance between the portions of the copper tubes which are in proximity with each other decreases, which shifts the current carried by the copper tube in the same manner which, in turn, shifts the magnetic field accordingly. Thus the magnetic field becomes more intense as the coil becomes smaller or the legs of the coil become closer together.
The applications of the alternating magnetic field heating of heating agents fall roughly into two general categories, designated for the purpose of this discussion, S and L.
Category S applications are characterized by some or all of the following features:
In such applications emphasis is placed on heating agents that either heat more slowly or are self-regulating in temperature. This leads to the selection of ferromagnetic, particularly metallic particles, that are small enough in size to either not be heated by or be heated inefficiently by eddy current action and thus rely primarily on hysteresis. Likely candidates would be smaller than, potentially considerably smaller than, 4 reference depths in size. Particles could be selected of a size that was marginally efficient in generating eddy currents at low temperatures and became very inefficient at higher temperatures as the particles'resistivity rose and permeability declined (and thus their skin depth increased).
Alternatively, ferromagnetic or ferrimagnetic heating agents could be selected that had Curie temperatures at or slightly above the desired temperature. These could be nonconductive heating agents such as ferrites, which relied on hysteresis, or conductive heating agents such as alloys with low Curie temperatures. When the Curie temperature is reached the permeability of the heating agent drops dramatically to approximately 1 and the material loses much of its ability to respond to a magnetic field, hence generate heat.
Category L applications are characterized by some or all of the following features:
In Category L applications for alternating magnetic field heating of heating agents, the rapid and efficient heating of the heating agents is of primary importance and the overheating or too rapid heating of the heating matrix is of less concern. Because the intensity of the magnetic field is frequently much less in these applications it is essential that these heating agents heat as rapidly as possible and efficiently transfer that heat to the materials they are designed to heat. Though most of these heating agents have high Curie temperatures and are capable of generating temperatures that would overheat or degrade the materials around them, their temperature can be regulated by controlling the heating time and/or field intensity.
The present invention is directed primarily toward applications more closely related to those of Category L in which a highly efficient and thus rapidly heating agent capable of quickly and evenly transferring that heat to the appropriate materials would be highly advantageous.
As noted above, in dielectric and microwave heating applications nonconductive, non-magnetic polar materials heat readily at those high frequencies (27 MHz-GHz). Numerous patents have been granted for applications in which the heating needed to be concentrated or intensified. They include Pitman U.S. Pat. No. 2,087,480 (1935) who added fine metal powder or filings to increase the rate of heating of an adhesive that would, without additives, heat dielectrically; seemingly countless patents that have added ferromagnetic or ferrimagnetic particles such as oxides and ferrites to containers and other objects for selective heating in microwave ovens; and patents in which conductive materials were added to adhesives and thermosets for use at microwave frequencies to enhance curing e.g. Wang U.S. Pat. No. 4,626,642 (1986) who found that graphite fibers heated too quickly and that steel and aluminum fibers heated more slowly than graphite but at rates equal to one another.
Metal screens have been used as heating agents in the lower radio frequency range where they relied on eddy currents. Alternatively, they were considered suitable by Kohler U.S. Pat. No. 2,393,541 (1943) at higher frequencies if they were small enough not to be heated by eddy currents and were of a composition with a suitable Curie temperature. Kohler also specified fine ferromagnetic particles (&lt;200-300 mesh or 74-50.mu.) that were too small to be heated by eddy currents and thus, because they were heated only by hysteresis, would heat only to a selected Curie temperature and therefore not overheat.
Guglielmo U.S. Pat. No. 3,620,875 (1971) specified: ferromagnetic particles 20-200 mesh (841-74.mu.) in size with the preferred range of 40-100 mesh (420-149.mu.); 50-66% loading in the heating matrix by weight; and 1-30 Kw generators operating at 5-50 MHz. Heating was predominantly by eddy currents, hence the large particle size and high frequency.
A number of patents (assigned to Heller) have relied on nonconductive, ferromagnetic oxides as heating agents. In the first, James U.S. Pat. No. 3,461,014 (1969) specified magnetic iron oxides sub .mu. to 20.mu. in size and 5-10 Kw generators operating in the 4-31 MHz frequency range. In Heller & Leatherman U.S. Pat. No. 4,035,547 (1977) the heating matrix had separate heating particles of non-conducting submicron iron oxides that responded to a high frequency magnetic field and agitating particles which were needle like and formed semi-permanent magnets that rotated in response to a second magnetic field in the audio range. The heating particles melted the heating matrix and substrate plastics and the agitating particles were sufficiently stiff to rotate in the audio field and cause an intermixing of the heating matrix and substrate plastics.
In Leatherman U.S. Pat. No. 4,969,968 (1990) the combination of submicron nonconductive iron oxide particles which heat by hysteresis and conductive ferromagnetic particles of at least one micron in size which heat by eddy currents were found to have a synergistic effect, resulting in unexpectedly higher heating rates than would have been predicted by the additive combination. It was noted that the eddy current heating of the conductive ferrous particles required large particles and high frequencies (5-30 MHz) while the submicron nonconductive particles operated in a much lower frequency range of 2-4 MHz. While the eddy current particles do generate some heat at the lower frequencies, it is at a significantly decreased rate and is "uniformly considered as undesirable and impractical". This is due to the skin effect phenomenon which adversely effects the eddy current formation and reduces the generation of heat. This is illustrated in the test tables where at 2.8 MHz a 60% loading of minus 200 mesh (&lt;74.mu.) iron powder had a heating rate of only 36% of the heating rate generated by a 60% loading of 20-200 (841-74.mu.) mesh iron powder.
The claimed frequency ranges for the multiple particle agent are: 500 KHz-5 MHz; 1.2-7 MHz; and 3.5-4 MHz. Test data was not included at lower frequencies, however the graphs indicate a marked drop in the heating rates at 2.8 MHz in comparison with 4.5 MHz, particularly at lower coil currents. This suggests that at lower frequencies the heating agent will become inefficient. It was noted that the preferred loading levels of 40% conductive powder and 25% nonconductive particles by weight, had a negative effect on the bonding strength, as would be expected with such a high loading level.
Leatherman indicated that an increase in the coil current increased the heating rate; more gradually for the eddy current responsive powder and more rapidly in the submicron nonconductive particles. "However the design requirements for the coil to make use of high current and frequency levels can be extremely difficult to implement." The tables and graphs indicate that a high coil current is required at both 2.8 MHz and 4.5 MHz to generate any significant heating of the submicron powders. Also of interest is the precipitous decline in the heating rates of all tested heating agents at lower coil currents. The graphs illustrating the relative heating rates of the heating agents indicate that for all the heating agents there is a minimum coil current below which no significant heating occurs. At 4.5 MHz that minimum is 250 coil amps and at 2.8 MHz that minimum is 300 coil amps.
The results observed by Leatherman parallel those of other patentees, the current applicant and industry experience. The results are in accordance with the relevant theories and can be briefly summarized as follows.
The Category S applications discussed above include heating agents which are specified to address the concern of heating that is overly rapid or generates too high a temperature. Frequently the heating agents are placed in high intensity fields associated with close coupling and/or multi-turn solenoid coils (FIG. 2 illustrates several multi-turn coils). Numerous patents relate to the welding of plastic pipe, the heating of heat shrinkable thermoplastics and related applications such as cable blocks. By selecting a ferromagnetic or ferrimagnetic material with an appropriately low Curie temperature the heating agent self-regulates to a selected temperature that does not degrade the heating matrix or adjacent materials.
In Monovoukas U.S. Pat. No. 5,378,879 (1995) flake or disk-like ferromagnetic particles with high conductivity and high magnetic permeability were specified. The intent was to provide high efficiency particles that could be loaded into a non-magnetic, non-electrically conductive material or matrix at low loading levels and generate heat rapidly in a controlled fashion. The small size and shape of the particles combined with their low loading levels did not change the desired properties of the article or material. Monovoukas notes that "electrically conductive ferromagnetic particles of a size several times larger than the particle skin depth may be efficient generators of heat by eddy currents. Small skin depth may be achieved with particles of high magnetic permeability and high electrical conductivity exposed to a magnetic field of high frequency." This is in keeping with the principles previously outlined above.
To this end, the preferred material was nickel with a calculated skin depth at 5 MHz of 6.2.mu. (or nickel alloys) and the preferred configuration was thin flakes because of their high surface area to volume ratio. The percentage loading by volume is preferably between 0.1% and 50%, more preferably between 0.5% and 10% and most preferably about 2%. It is noted that articles loaded above 15% by volume are generally not preferred, and, in fact, are achievable only with particles having relatively lower aspect ratios. Numerous ferromagnetic and ferrimagnetic materials in the logical shapes of flakes, fibers, powders and spheres were loaded at 5% by volume into low density polyethylene and formed into rods 7.9 mm in diameter and 58 mm in length. These were inserted into a 14 turn solenoid coil 11.2 mm in diameter and 73 mm in length, operated at a frequency of 4 MHz with a 30 amperes rms. current. The tests indicated that the high surface area to volume nickel flakes with a major dimension of 30.mu. (therefore more than 4 times the calculated skin depth of 7.mu.) heated significantly faster than any other alternative. Other shapes and materials with dimensions several times their skin depth also heated well.
In a patent related to gluing together two non-metallic substrates with a hot polymerizable adhesive Berce U.S. Pat. No. 5,447,592 (1995) stressed "the uselessness, even the disadvantage, of having over rapid heating of the ferromagnetic or ferrimagnetic fillers in a medium that has low thermal conductivity" and that the particles not be overly large such that they overheated and spoiled the medium. To this end small particle sizes (1 .mu.m.sup.3 -100,000 .mu.m.sup.3) were claimed and much smaller particles (1 .mu.m.sup.3 -1500 .mu.m.sup.3 or 1000 .mu.m.sup.3 -1500 .mu.m.sup.3) were preferred in the text. The samples glued in the tests consisted of two substrates 25 mm in width and 3 mm in thickness with a glued area of 25 mm.times.25 mm and a glue thickness of 2 mm. They were placed in a solenoid induction coil 40 mm in length with six oval spirals or turns 20.times.30 mm in size, supplied by a 6 Kw generator operating at 200 KHz. The shapes and composition of the heating agent materials paralleled those tested in Monovoukas U.S. Pat. No. 5,378,879 (1995) including magnetite and ferrite spheres and nickel powders and flakes. Given the low frequency (resulting in large skin depths) and small particle size the heating was predominantly due to hysteresis losses.
While the prior art Category L processes for induction heating of matrices via heating agents have found a niche in the market they have numerous short comings which have limited their market acceptance. Solutions to these short comings would greatly increase the market for induction heating of matrices via heating agents and make it possible to produce end products which heretofore have been either impractical or economically unviable, as well as significantly increasing the production rates and profitability of existing applications. Many of the short comings outlined below reflect long felt and well recognized needs in the industry that prior art technologies have been unable to solve or fulfill.
Short Comings of the Prior Technologies Include:
1) Long Cycle Times and/or Low Production Rates.
In commercial applications production rates are frequently critical and can determine whether a particular production process and/or in some cases whether the product itself is economically viable. If the induction heating cycle time is too long, another process will be selected. If no other process is viable the product is not viable. This calls for highly efficient heating agents that can heat rapidly and transfer that heat to the matrix and adjacent substrate quickly without degrading either. Faster cycle times can increase the production on existing equipment. More efficient heating agents would allow the use of more than one coil or a multiple station coil (FIG. 1.c) thereby increasing production, particularly if the heating agents maintain their efficiency at the lower frequencies and coil currents associated with longer coils.
2) Inability to Overcome Poor Coupling.
With some parts it is not possible to get the coil close to the heating agent or on both sides of the heating agent and as a result either the application is not possible or it is not viable because the field is too weak and therefore the cycle times are too long. For example, this can be due to part geometry and wall thickness. A significantly more efficient heating agent could make previously impossible applications possible and shorten the cycle times sufficiently to make other applications practical.
3) High Power Input Requirements.
As noted in the discussion regarding Leatherman U.S. Pat. No. 4,969,968 (1990) because the prior art heating agents are inefficient, particularly at lower frequencies, in many applications it is necessary to use high power generators that are capable of generating sufficient coil current. Such generators are large, expensive and relatively inefficient. A higher efficiency heating agent would eliminate this requirement and make it possible to use smaller, less expensive generators.
4) Required High Percentage Loadings.
Because the prior technologies' heating agents are inefficient generators of heat, their required loading levels are high and this significantly degrades the strength of the heating matrix. More efficient heating agents would require lower loading levels in the matrix and therefore would retain more of the matrix's original strength.
5) Limitations to Matrix Modifications or Enhancements.
The high percentage loadings required by the prior art to achieve adequate heating rates, and the attendant loss of strength caused by those high loadings, limit the characteristics of the base matrix and the option of introducing additives to modify or enhance the matrix. For example, in the case of thermoplastics, thermosets and adhesives this limits the ability to utilize higher molecular weight or viscosity materials, due to processing problems. It also limits the ability to introduce additives which could, for example: increase the matrix's tensile strength (carbon, glass or other high strength fiber); conductivity (carbon black, carbon fiber, nickel coated carbon or glass fiber, drawn stainless steel fiber etc.); flame or smoke retardance; and other materials for alternative enhancements. A high efficiency heating agent that required lower loadings would make it possible to utilize these modifications and enhancements.
6) Inability to Produce an Effectively Transparent or Colored Matrix.
The high loading levels of the prior art heating agents make the matrix opaque and give it a distinct color (rusty in the case of ferromagnetic oxide and gray black in the case of ferromagnetic powder). This has proven to be a problem where the substrates are transparent or translucent and therefore the heating matrix is visible. A heating matrix that requires only low loading levels, could produce a matrix that is effectively transparent or easily colored.
7) Localized Overheating.
Large ferromagnetic powders form hot centers which lead to localized overheating and degradation of the matrix. A more efficient heating agent would transfer heat more quickly and evenly to the matrix, thus eliminating the degradation.
8) Migration of the Particles During Heating.
When the ferromagnetic particles form hot centers they overheat the matrix adjacent to them causing a localized decrease in viscosity and allowing them to rapidly migrate. In some cases they migrate outside the desired area where they can arc to the coil, causing damage to the substrate or the coil. In other cases they tunnel through the substrate, sometimes many millimeters, thereby perforating or contaminating the substrate. A heating agent that did not tend to migrate would minimize these problems.
9) Problems in Getting the Heating Agent Into or Keeping it in suspension.
The ferromagnetic oxide based heating agents are difficult to put into suspension in a liquid or paste without agglomeration. The agglomerations tend to form significant hot spots and uneven strengths. This problem adds to the cost of manufacturing the heating agent. The ferromagnetic powder based heating agents are difficult to keep in suspension because of their high density or more specifically their high apparent density or apparent bulk density. This results in variable loading levels in the heating matrix and difficulty in assuring acceptable heating rates. If the loading level is too high the matrix over heats but if the loading level is too low the matrix does not get hot enough. A heating agent that could be put into suspension and maintained there with relative ease would be highly advantageous.
10) Limited Recyclability.
The prior art iron oxide or iron powder based heating agents cannot be readily separated from the matrix and therefore contaminate the recycled material. For example, in the case of thermoplastics if a substrate is to be recycled the prior art heating material must be cut out by hand. While this is problematic for recycling at the end of the substrate's useful life it is particularly problematic for the manufacturer who seeks to regrind and reuse substrates not meeting quality standards. Either not recycling the substrate or cutting out the heating agent by hand significantly increases the manufacturer's cost and puts the prior art technologies at a competitive disadvantage compared to alternative technologies that do not require heating agents. Thus a heating agent that could easily be separated from the recycling stream would have distinct competitive advantages, particularly in an increasingly environmentally conscious era.
11) Power Input Rate Limitations.
Because the iron powder cannot transfer its heat quickly to the matrix the power input must be limited to minimize matrix degradation. The ability of a more efficient heating agent to rapidly transfer its heat allows the use of higher power, hence faster cycle times, without degradation.
12) Inability to Heat High Temperature Materials Adequately.
Certain materials such as engineering plastics require high temperatures and high strength welds or joints. Because the prior technologies are inefficient generators of heat they are limited in the temperature they can attain. Adding a higher loading of the prior art heating agent increases the heating rate but significantly degrades the strength of the heating matrix.
13) Inability to Generate Sufficient Heat Rapidly Enough to Selectively Heat the Matrix Adequately Before Overheating the Adjoining Conductive Substrate.
For example, carbon fiber reinforced substrates heat in an alternating magnetic field. Frequently the carbon fiber reinforced substrates are either closer to the coil and/or between the coil and the heating agent. This subjects the substrate to a significantly more intense field than the heating agent. Therefore the heating agent must be highly efficient so that it reaches the required temperature before the substrate overheats and causes distortion, delamination or degradation of the substrate.
14) Limits to Coil Length.
As previously discussed, as coil lengths increase: induction increases and the frequency drops; and resistive losses increase causing a decrease in the coil current. With the inefficient prior technologies these results lead to excessively long cycle times and in some cases the inability to generate adequate heat at all. Numerous potential applications using long coils have not proven possible or practical. With the prior technologies an alternative, in some cases, is to utilize two generators and two coils, thereby increasing the cost and significantly increasing the complexity and difficulty. A highly efficient heating agent could still generate sufficient heat at the lower frequencies and coil currents to make the large coil applications practical and in other cases to eliminate the second generator and coil.
15) Frequency Limitations and the Resultant Inefficient Utilization of Generator Capacity.
When the frequency becomes too low the prior technologies become very inefficient generators of heat, frequently to the point that they are too slow or cannot reach a sufficient temperature at all. Not only is the frequency drop a problem for long coils, in many applications it is not possible to tune the generator to provide full output because in doing so the frequency would drop to a point that the prior heating agents do not heat sufficiently. As a result a generator may be tuned to only 50 or 60% of its output, which slows the cycle times and may require a larger, more expensive generator. A heating agent that generated sufficient heat at lower frequencies would make long, poorly coupled coil applications viable. In addition it would allow the generator to be tuned to a higher output, thereby either allowing the use of a smaller, less expensive generator or decreasing the cycle times and increasing the throughput of an existing generator.
16) The necessity of using vacuum tube generators and the inability to use solid state generators.
The prior technologies require frequencies in the megahertz range which can only be attained with vacuum tube generators. A high efficiency heating agent that generated adequate heat in the kilohertz frequency range would have industry shaking advantages. It would make it possible to utilize modern, solid state generators which have marked advantages over the vacuum tube generators.
17) Inability to Provide a Non-corrosive/non-reactive heating agent with commercially Acceptable Heating Rates.
The heating rates for stainless steel powders are significantly lower than the heating rates of iron powder. Increasing the size of the stainless steel powder and/or its loading levels has only helped slightly. There are numerous applications where the use of non-corrosive/non-reactive heating agents would either be highly advantageous or essential, if the heating rates and cost were acceptable.
18) Inability to Generate Sufficient Heat in the Heating Matrix, Rapidly Enough, to avoid distorting the adjacent substrate.
A highly efficient heating agent is capable of bringing the heating matrix and the adjoining surface of the substrate to the required temperature very rapidly before other portions of the substrate are overheated and distorted. Less efficient prior art heating agents, given considerably more time, may ultimately bring the matrix to the required temperature but throughout that extended time heat is migrating through the substrate, leading to its overheating and/or distortion.